IIRC even the Mythbusters proved that the Spontaneous Combustion idea is false.

What occurs is usually the person either dies or is asphyxiated due to smoke inhalation in an enclosed room. If the person was smoking in bed or elsewhere, their clothing or other material catches fire which quickly extinguishes itself due to the lack of Oxygen in the room(enclosed area). The fire them slowly smolders. a body will burn slowly in this manner for several days leaving at best the hands and feet. THe event looks like the person spontaneously combusted. However their combustion was a much more delayed action and not spontaneous at all.In a rich man's house there is no place to spit but his face.
Diogenes Of Sinope

coroners in many places do not have to be Pathologists. many rural areas of the US have the local mortician working as a coroner. A Pathologist would hopefully not fall prey to this myth. It is true that the actual cause of death is quite obfuscated by the body being burned. However, the problem with the myth is that there are no eye witnesses to the burning as it occurs. Given that little conundrum time to sink in and a reasonable person will conclude that either its a coincidence or that spontaneous combustion is a crockIn a rich man's house there is no place to spit but his face.
Diogenes Of Sinope